To operate internal combustion engines, fuel is injected into cylinders and a corresponding mass flow rate of air is supplied. The mass flow rate of air into the cylinders, which represents the so-called air charge of the cylinders, is adapted to the specific engine state. A setpoint air charge of the cylinders is determined by a torque requested, for example, by the driver; with the aid of a throttle-valve control system, the mass flow rate of air into the engine being controlled, and it being ensured that the actual air charge follows the setpoint air charge as effectively as possible. This is complicated by the fact that not only does the throttle valve have to be set to allow the desired mass to flow into the engine, but also that the volume of the vessel between the throttle valve and engine, the so-called induction pipe, must be filled or emptied as rapidly as possible, i.e., a desired induction-pipe pressure must be built up or reduced.
The volume of the induction pipe causes a requested setpoint air charge to not immediately correspond to the actual air charge, i.e., the actual mass flow rate into the engine. This behavior is normally taken into account by a so-called charge controller, which ensures that the actual air charge builds up with the same dynamic response to the greatest possible extent. In the case of engines controlled with the aid of the throttle valve, this achieves reproducible behavior and rapid response of the engine.
In the case of forced-induction engines, i.e., internal combustion engines having a charging device, such as an exhaust-gas turbocharger, supercharger or similar charging devices, the air charge cannot be adjusted in every operating state with the aid of the throttle valve. In a throttle-valve mode, it may be provided that the pressure in the charge-air line in front of the throttle valve be greater than the pressure corresponding to the desired setpoint air charge, the throttle valve being operated in an adjustment range, in which the pressure (built up by the mass flow rate of supplied air) is throttled away in the charge-air line, in order to set an air charge (pressure) in the induction pipe that is necessary according to the charge controller.
To this end, however, it must be ensured that a sufficiently higher pressure (charge-air pressure) is always available in front of the throttle valve than is intended to be set in the induction pipe in back of the throttle valve, which means that the fuel consumption is increased.
In a charging mode with higher requested air charges, the boost-pressure control system may provide the air charge, the throttle valve being completely open or only slightly throttled. To this end, in the event of a correspondingly high setpoint air charge, the charge control system calculates a setpoint boost pressure, which is then set, in turn, by the boost-pressure control system (of the charging device). A classical boost-pressure control system is made up of, for example, a precontrol unit and a characteristics-map-based PI(D) controller. The precontrol unit is normally operated in a steady-state manner and does not take into account that an induction pipe having a specific volume is situated between the charging device and the engine. In the characteristics-map-based controller, there is no direct relationship between the characteristic dynamic response of the system (i.e., the physics describing this vessel) and the control parameters. Consequently, the characteristics map may also not be readily used for setting a predefined dynamic response characteristic of the charging in the pressure-charged region.